Heretofore, the process for producing isobutene-cyclodiene copolymers has been well known. In order to appreciate the commercial utility of the copolymer as a rubber, the copolymer should possess appropriate mechanical strength and strong adhesive strength as well as a high number-average molecular weight and a high degree of unsaturation.
It is known that as the number-average molecular weight of the copolymer increases, the copolymer's tensile strength tends to increase, and as the number of unsaturation bond of the copolymer increases, the copolymer's adhesive strength with respect to another rubber tends to increase. In other words, when the copolymer is vulcanized together with highly unsaturated rubber such as natural rubber etc., the adhesive strength of the copolymer increases as the number of unsaturation bond increases and the crosslinking reaction increases and the similarity in crosslinking behavior will also increase.
Isobutene-isoprene copolymer ("butyl rubber") is well known as a representative example of existing isobutene-diene copolymer. But butyl rubber has less than 2.5 mol % of isoprene content as comonomer, which has double bond; thus, it has low number of crosslinking sites which can bond with other rubber. Also its crosslinking reaction behavior is different from the highly unsaturated rubber. All of the above for example, results in its weak adhesive strength which decreases further when exposed to external shock, vibration etc. As a method of improving adhesive strength of butyl rubber, it has been proposed to incorporate halogen compounds into butyl rubber, such as chorine and bromine which can promote the crosslinking reaction, and to increase isoprene content. But in case of the former, investment on additional equipment is required for halogenation process of the resulting polymer after the production of the polymer. In case of the latter, although improved adhesive strength is obtained, there exists the problem of decreasing gas barrier property, which is one of the most desirable properties of butyl rubber. Moreover, U.S. Pat. Nos. 3,356,661, 3,165,503 and 3,466,268 etc. reported that the higher the content of isoprene, the lower the number-average molecular weight and as a result, the copolymer of little utility value is produced. Also it is known that the structure and quantity of the unsaturated bond influence the resistance of rubber against aging. In case of butyl rubber, the degree of unsaturation is lower than that of natural rubber, so it is somewhat stable against aging, but because the site of unsaturation is in the polymer backbone, this polymer is subject to ozone cleavage, thus the aging of rubber cannot be avoided.
On the other hand, in case of isobutene-cyclodiene copolymer which is similar to butyl rubber, improvement in adhesive strength is obtained as well as excellent gas barrier property even at high degree of unsaturation. Even if the unsaturation bond is attacked and cyclic structure is severed, the copolymer backbone will be highly resistant to ozone attack because the diene compound having cyclic structure is copolymerized and the unsaturation bond does not exist on the backbone. Thus the resistance of rubber against aging is excellent and its improved characteristics makes it an excellent tire material. Even though the above-mentioned isobutene-cyclodiene copolymer solved the problem associated with existing butyl rubber and are considered excellent material for tires, the copolymer faces another problem in that it is difficult to maintain the comonomer in high purity because the comonomer is unstable against heat. Also as the degree of unsaturation increases, the gel formation increases and the molecular weight decreases. These problems in preparation have prevented said polymer from being producted for commercial use.
Among processes for preparing isobutene-cyclopentadiene copolymer of the prior slurry polymerization methods, U.S. Pat. No. 2,356,128 produced only copolymers of low molecular weight. U.S. Pat. No. 2,577,822 disclosed a process for preparing terpolymer having high molecular weight which used a divinylbenzene to give crosslinking. U.S. Pat. No. 3,080,337 disclosed a process for preparing isobutene-cyclopentadiene-isoprene terpolymer without gelation and U.S. Pat. No. 3,239,495 disclosed a process for preparing terpolymer of high molecular weight using divinylbenzene.
However, these slurry processes produce a polymer of low molecular weight due to cyclopentadiene dimer (DCPD) and water which were included in their reaction materials, or a polymer of poor physical properties which cannot be used as a commercial product due to the addition of divinylbenzene which increased the crosslinking of the polymer produced.
Although isobutene-cyclopentadiene copolymers having a useful degree of unsaturation and molecular weight can be produced by effectively removing water and dicyclopentadiene from the reaction materials, problems such as gelation and fouling were experienced as the degree of unsaturation and molecular weight of the copolymer increased. Particularly, in a slurry batch polymerization process, it was impossible to produce a polymer having a homogeneous degree of unsaturation because in the initial stage of the polymerization reaction, when cyclopentadiene having a high reaction rate was used, a polymer having a high degree of unsaturation was obtained and gelation was observed.
By conducting a slurry continuous polymerization together with the removal of impurities which would decrease the molecular weight of copolymer, isobutene-cyclopentadiene copolymer without gelation can be produced. However, it was difficult to prevent reactor fouling in a slurry continuous reaction. The technique to extend continuous operation time by preventing fouling is an important index to measure the economical performance of a slurry process.
The problem with gel formation and molecular weight decrease can be solved by the well known solution process, however, it is impossible to obtain high conversion because the viscosity of the reactants increases sharply when the polymerization proceed. Also, in order to obtain high molecular weight of the polymer, the production cost together with the investment cost will increase since the temperature has to be maintained at a level of -120.degree. C.
A number of examples of process for producing isobutene-cyclodiene copolymers by solution process are shown below:
In U.S. Pat. No. 3,808,177, an isobutene-cydopentadiene copolymer having a number-average molecular weight of at least 120,000 and a degree of unsaturation of 8.about.30 mol % at no more than 10% of conversion, was prepared by carrying out a polymerization reaction using aluminium chloride dissolved in methyl chloride as catalyst and aliphatic saturated hydrocarbon with 5.about.10 carbon atoms as reaction solvent at polymerization temperature of -120.degree. C.
In U.S. Pat. No. 3,856,763, an isobutene-cyclopentadiene copolymer having a number-average molecular weight of at least 120,000 and a degree of unsaturation of 8.about.40 mol % at no more than 10% of conversion, was prepared using the chioro or bromo allyl aluminium dihalide with alkyl groups of 1.about.4 carbon atoms as catalyst at polymerization temperature of -120.degree. C.
In U.S. Pat. No. 4,031,360, an isobutene-cyclopentadiene copolymer having a number-average molecular weight of at least 90,000 and a degree of unsaturation of 8.about.35 mol % at no more than 10% of conversion, was prepared by carrying out a solution polymerization method using aluminium halide or alkyl aluminium dihalide as catalyst at polymerization temperature of -120.degree. C.
In U.S. Pat. No. 4,139,695, an isobutene-methylcyclopentadiene copolymer having a number-average molecular weight of at least 120,000 and a degree of unsaturation of 8.about.30 mol % at no more than 5% of conversion, was prepared using alkyl aluminium dichloride as catalyst and methylcyclohexane as reaction solvent at polymerization temperature of -120.degree. C.
Even though an isobutene-cyclodiene copolymer, including cyclopentadiene of which the number-average molecular weight and the degree of unsaturation are high and gel content is low, can be prepared by the solution process, this solution process has several problems as shown below.
In solution process, since the produced copolymer is dissolved in the reaction solvent, the viscosity of the solution increases rapidly as the reaction proceeds, so that the homogeneous mixing of the solution is impossible, and a large amount of power is used for stirring the solution. Also, since the viscosity of the solution increases, it becomes difficult for the solution which serves as a medium for removing reaction heat to transfer the heat to refrigerant and to control the reaction temperature. In particular, a temperature gradient inside the reactor occurs by local temperature increases, so that it is difficult to prepare polymers having homogeneous physical property. Therefore, there is a high risk of producing low quality products.
For preparing polymers having a number-average molecular weight of at least 100,000, the increase of viscosity becomes a bigger problem since the polymerization must be carried out at a extremely low temperature of -120.degree. C. compared to that of slurry process. Further, in order to maintain the low temperature, very large cooling capacity is required, and the amount of refrigerant used must be increased, resulting in an increase of the production cost.
Also, to keep the solution in homogeneous phase at lower polymerization temperature, the required amount of reaction solvent becomes larger; however, the problem of producing lower molecular weight polymers becomes more severe with the increase of the amount of solvent used.
Therefore, in order to overcome the above mentioned problems, the conversion of reaction must be kept as low as 10%. However, in this case, due to the rapid increase of amounts of reactant and solvent which are recycled, the distillation unit and moisture purification unit would have to be enlarged, resulting in increase of the production cost. Further, when conversion of reaction is maintained at a low level, there is a burden to separate and recycle the comonomer since cyclopentadiene does not completely react at low conversion. And, since the reaction solvent is not readily volatilized, high temperature is required for degassing the residual solvent, impairing the double bond in the produced isobutene-cyclodiene copolymer and deteriorating the copolymer's physical property.